1. Field of the Invention
The present invention is generally in the field of electrical circuits. More specifically, the present invention is in the field of oscillators and tuning circuits.
2. Background Art
Voltage controlled oscillators (VCOs) can be utilized to provide operating frequencies for communications devices, such as cell phones, as well as other types of electronic devices. VCOs in communications devices, such as cell phones, can typically be adjusted to provide a range of output frequencies, thereby enabling the communications devices to support multiple frequency bands for communications standards such as Global System for Mobile Communication (GSM), Extended GSM (EGSM), Digital Cordless Standard (DCS), and Personal Communications Service (PCS), and Wideband Code-Division Multiple Access (WCDMA). However, with the addition of frequency bands VII and XI and frequency bands for communications standards such as Long Term Evaluation (LTE), it is increasingly more difficult for VCOs in the communications devices, such as cell phones, to provide the increased frequency range required to support the additional bands.
In one conventional solution, multiple VCOs are incorporated in a communications device, where each VCO handles a portion of the required frequency range. However, the multiple VCOs undesirably require significantly more semiconductor die size than a single VCO. In another conventional solution, the capacitance of an adjustable coarse tuning capacitors in the tank circuit of the VCO is increased to allow the VCO to cover the required frequency range. However, since the quality factor (Q) of the tank circuit is inversely proportional to the square root of the tank circuit capacitance, increasing the capacitance undesirably lowers the Q of the tank circuit. Also, by lowering the tank circuit Q, VCO phase noise, which is inversely proportional to the Q of tank circuit, is undesirably increased.